As is known, an exhaust catalytic converter only performs its task of reducing the unburnt hydrocarbons, carbon monoxide and oxides of nitrogen content of the exhaust gases after it has reached a critical temperature, termed the light-off temperature, which is between 300.degree. C. and 400.degree. C. In the case of an internal combustion engine in which very lean mixtures are burnt under light load conditions, the excess air tends to cool the catalytic converter making it difficult for the latter to remain above the light-off temperature under certain conditions, notably during idling. To some extent, this problem can be counteracted by placing the converter nearer to the engine (close coupling) but the converter may then not be able to withstand the temperature at full load. Ideally therefore, the converter should be kept sufficiently far away from the engine to allow safe full load operation, but means should be provided for heating the catalyst when it risks dropping below its light-off temperature.
Diesel engines tend to produce soot in their exhaust gases and are required to have a filter trap in order to meet regulations for reducing toxic emissions from engines. Such filter traps tend to be blocked by the soot particles after some time and various proposals have been made for regenerating the filter trap. One way is to burn off the soot and it is known that if burning is started by an external heat source at the front face of the filter trap, the exothermic reaction will itself spread along the length of the trap in a manner similar to the burning of a cigarette when air is drawn through it.